Extrusion-type pellet mills and the process of pelleting particulate feed material using such devices is old in the art. The processing of particulate feed material is, nevertheless, quite complex. In order to process feed efficiently, it is important to take many factors into account during the design and operation of the system. Pelleting has been defined as an extrusion-type thermo-plastic molding operation in which finely divided particles of feed are formed into a compact, easily handled, pellet. The process is thermo-plastic because the protein and sugar components in the feed become plastic or moldable when heated and diluted with moisture. Pellet mills are designed to compress feed and force it through extrusion passages in an extrusion die when the moist feed has been heated. During this process, the feed is compressed and molded, and pellets are formed.
Pelleted feed is known to be highly desirable because it, among other things, allows better utilization or conversion of the feed ingredients, prevents selective feeding on favored ingredients, increases density, and provides better feed handling characteristics which simplify bulk handling.
Pellets are generally formed in a pellet mill when the feed is compressed between a pressure roller and an extrusion die. All the other parts of the pellet mill simply facilitate the continuous compression of feed between the pressure rollers and the die and the handling of the extruded pellets. Each pellet mill is generally equipped with a die and roller assembly which often includes a plurality of pressure rollers, an extrusion die, and a mechanism for delivering feed material evenly along an inner surface of the extrusion die so that the feed can be compressed by the pressure rollers when they roll over the inner surface or compression surface of the die. Each roller assembly generally has an optimum efficiency velocity depending upon the sieve analysis or density of the feed being pelleted and the characteristics of the die and roller assembly. Obviously, it is desirable to produce as much pelleted feed material as possible over any particular period of time. However, the design of the roller assembly and its various parts, create limitations to the production efficiency of any particular roller assembly. Much of the effort to improve roller assembly design has been concentrated on the design of the pressure rollers.
Landers (U.S. Pat. No. 2,875,709) discloses a dimpled pressure roller for a pellet mill which is designed to improve the traction of the pressure roller and, therefore, the force of the pressure roller upon the feed material in relation to the contact point with the extrusion die. This force acts to compress the material and extrude it and is called the roll force.
In spite of various efforts to improve the traction of the pressure rollers, limitations still existed with respect to the velocity at which the pressure rollers compress the feed on the inner surface of the die. If the velocity is too great, air entrained in the feed will be driven out of the feed ahead of the roller as the roller rolls along the inner surface of the extrusion die. This air will push the feed away from the roller and cause a build up of feed in front of the roller so that it exceeds the intended depth of feed in front of the roller in the feed wedge. Unfortunately, there is generally a large amount of air entrained in the feed and it must go somewhere. This is especially true for feed materials, such as materials intended for newborn animals which may contain large amounts of whey and may have densities below about 32 pounds per cubic foot. The faster the roller turns over the compression surface of the die, the faster the air is forced out of the feed. When the roller turns too fast, the feed builds up in front of the roller. This is a classic problem which generally limits the speed at which pellet mills can be operated. When the build up of feed in the feed wedge is too great, the roller often slips on the feed wedge forcing the wedge to squirt along the inner surface of the extrusion die in front of the roller. This event is called a roll slip. Rather than rolling over the feed, the roller pushes the feed ahead. When this occurs, the tangential force of the feed wedge sliding across the die surface, wears the die, thereby reducing its useful life. Production may even need to be shut down because of the heat produced by the friction between the wedge and the die surface, which can result in a fire.
It will be appreciated that any improvement in the design of a pellet mill, or any of its parts, which reduces the potential for a build up of feed in the feed wedge, thereby lessening the tendency to cause roll slips, will reduce the frictional wear on the surface of the extrusion die, thereby lengthening its useful life. It will be further appreciated that this problem limits production efficiency because the operating velocity must be limited. Other problems also exist which are in need of solutions. The present invention provides solutions for these and other problems associated with pelleting and pellet mills in general. In addition, the present invention offers other advantages over the prior art, and solves other problems therewith.